World Cup fans heading to Rio de Janeiro might want to think twice about swimming in the sea - as these shocking photos reveal the true extent of its sewage problems.

More than 400,000 tourists are expected to descend on Brazil's 'Marvellous City' next month, but they might want to avoid the ocean.

Local photographer Eliseu Cavalcante snapped the destination's waterways - which are pumped with human excrement.

Raw sewage: Eliseu Cavalcante's photos show the extent of the sanitation problem in Rio where human excrement is pumped into the sea. A mixture of cyanobacteria and sewage flow off the coast of Barra Beach

Warnings: Raw sewage flows onto Sao Conrado Beach - an upmarket area but where it's unsafe to swim due to the sanitation problems

Only 40 per cent of sewage is treated in Rio - and the rest ends up in its rivers, lagoons and bays.

And as these images show, it also ends up floating around on and near the beaches where tourists will be flocking.

Share this article

Cavalcante told website Rio Gringa that she wanted to raise the issue of sanitation at a time when more foreigners are paying attention to Brazil.

Health concerns: Biologist Mario Moscatelli has been doing flyovers of Rio¿s waterways since the late 1990s to document the city's environmental degradation

Health warning: Tires and sewage float in Guanabara Bay near Rio¿s international port

According to Global Post, the government plans to clean up the usually picturesque Guanabara Bay - where up to 100 tonnes of rubbish are dumped every day - ahead of the 2016 Summer Olympics - especially because of the water events involved.

Mario Moscatelli, a biologist who's been monitoring Rio’s waterways for decades, said more than £595m had been spent trying to clean up the bay in the past 20 years, but the situation has worsened.

'Not in my worst nightmares would I have imagined that the Brazilian authorities would have done this with the environment,' he said.

A contaminated lagoon in Rio's Barra has different colours due to sediment, sewage flows, and the resulting cyanobacteria - some of which are toxic

Unsafe for swimming: A sunbather sits on the beach near Rio's Niteroi as trash and sewage float by

Rio's famed 'City of God' slum shows raw sewage is part of life

According to the city government's environment agency, 12 locations along beaches in Zona Sul — the most popular area for tourists - are unsafe for swimming due to the poor water quality.

Dr Daniel Becker, founder of the nonprofit Center for Health Promotion in Rio, said: 'In Rio, if you're going to the beach, you're going to sewage.'

He said exposure to raw sewage can cause illnesses such as diarrhoea and microbial diseases.

In April, the state government said it was cutting the Olympic clean-up budget from $1bn to £51m, according to Global Post.